Hitherto, a closed type cabinet, a bass-reflex type cabinet and a non-back baffle type cabinet have been known as a speaker cabinet. In the conventional cabinets, standing waves are generated by resonance of air inside the cabinet. The standing waves cause various problem as follows:
(1) The standing waves interrupt movement faithful to signals of a diaphragm of a speaker unit. As a result, an output sound frequency characteristic is disturbed and reproduced sound is distorted.
(2) A wall surface of the cabinet resonates with standing waves to produce sounds of the cabinet. A reproduced sound accompanied by the sounds of cabinet is different from an original sound. Also, the reproduced sound sounds as if it winds round the cabinet and has little exposition.
(3) A resonant sound of the standing waves leaks through the diaphragm of the speaker unit or a wall of the cabinet. Consequently, a specific sound of the reproduced sound has reverberation and is not clear. For solving these problems, various proposals have been made.
For example, in regard to the above problem (1), there have been proposed a method for filling up the cabinet with a sound absorbing material; a method for using a perforated sound absorbing board (Japanese U.M. Examined Utility Model Application Publication [JITSUKOSHO] No. 49-27241); a method for using a perforated sound absorbing board together with a sound absorbing material (Japanese U. M. Unexamined Utility Model Application Publication [JITSUKAISHO] No. 54-3930); a method for setting a sound reflecting board (Japanese U. M. Examined Utility Model Application Publication [JTTSUKOSHO] No. 57-49492); a method for using a honeycomb core together with a sound absorbing material (Japanese Examined Patent Application Publication [TOKUKOSHO] No. 58-45236); and a method for providing a sound absorbing cabinet inside the cabinet (Japanese Examined Patent Application Publication [TOKUKOSHO] No. 61-61597). For the problem (2), it has been proposed that walls of a cabinet are thickened. For the problem (3), a method for covering an opening of a frame of a speaker unit with a sound absorbing material has been known.
However, these proposals have the following disadvantages.
(1) In case the cabinet is filled with the sound absorbing material, a lively, dynamic sound can not be reproduced. With the perforated board, only a special standing wave having a wave length corresponding to a size of the perforation can be absorbed to thereby have little absorbing effect. Thus, a plurality of standing waves is generated in the cabinet, which results in no practical use. In case the performed board is used together with the sound absorbing material, various problem accompanied by mal-effects of the sound absorbing material arise. In case the sound reflecting board is used to scatter a sound it is necessary to select a material, size, shape and setting direction of the sound reflecting based according to a direction and a wave length of the Standing wave. Thus, it is difficult to cope with many standing waves. Further, in case an uneven diffusion reflecting structure comprising the sound reflecting board is formed on an inner surface of the cabinet, an effective volume of the cabinet decreases. The honeycomb type core has a large total opening area so that it has an effect only for a specific sound. When the honeycomb core is used together with a sound absorbing material, a reproduced sound is accompanied by a mal-effect of the sound absorbing material. It has been proposed that a sound absorbing cabinet is provided in tie cabinet. However, the purpose is to raise a reproduced sound pressure level by resonating with a sound of specific frequency. The sound absorbing cabinet having small internal loss makes a reproduced sound an unnatural sound accompanied by a reverberation, in resonance with a sound having a specific frequency, if used in a wide range sound reproducing speaks system.
(2) In case the wall of the speaker cabinet is thickened, since the standing waves is are not suppressed the problem of the diaphragm of the speaker unit an not be solved.
(3) In case the opening of the frame of the speaker unit is covered with the sound absorbing material, a large quantity of the sound absorbing material causes a mal-effect to a tone quality. Also, it is difficult to eliminate mal-effects of the standing waves having frequencies less than 300 Hz with only the sound absorbing material.
Accordingly, the present invention is to provide a speaker system which can reproduce a clear bass, sound and a stereophonic sound having an expanded sound field and a method for improving sound quality thereof.